component: add hero-split page block

[Ducksss]

Context

The roadmap names hero-split as a Hero family variant, and hero-basic already proves the family shape with shared heroFields.

This should be a full Payload page block, not a file-only post component.

Scope

Add hero-split as an installable page block variant that composes the existing Hero shared field base where appropriate.

Acceptance

• Source exists under payload-components/source/blocks/HeroSplit/.
• Manifest exists at payload-components/manifests/hero-split.json with sampleContent.
• payload-components/registry.json includes hero-split.
• Docs page exists at content/docs/components/hero-split.mdx using the standard component doc shape.
• src/lib/site.ts registers the component and updates any family metadata.
• Demo twin is registered and passes the class-mirror guard.
• Installer coverage proves payload-components add hero-split wires Pages + RenderBlocks and reruns cleanly.

Out of scope

• Video backgrounds.
• Dramatic/animated hero variants.
• New shared Hero abstractions unless real duplication appears.